Electrostatic charges are created when dielectric materials are moved relative to each other or to other materials. Electrostatic charges are critical particularly in applications involving fast moving, thin sheets of material, such as paper or foils. If these electrostatic charges are not reduced or neutralised, they can lead to uncontrolled discharges. Discharges of such kind can injure people, damage materials and cause sparks, fire and explosions. In addition, downstream processes such as coating, printing and finishing are impaired. This can also affect product quality significantly, even to the point of complete destruction of the product or material.
An active electrode assembly comprising a plurality of active needle-shaped individual electrodes that are electrically connected to a high voltage source and supplied with alternating current when an antistatic device is in operation is known from DE 1 197 1 342 A1. In this case, the electrode assembly is arranged in a bar-shaped electrode carrier in which a ground conductor is also integrated.
From U.S. Pat. No. 6,674,630 B1, antistatic devices are known in which either two active electrode assemblies with the same polarity are arranged consecutively in the direction of movement of a material web, or in which two pairs of electrode assemblies are arranged consecutively in the direction of movement of a material web, each pair comprising a positive electrode assembly and a negative electrode assembly that are arranged consecutively in the direction of movement of the material web. In these known antistatic devices, the neutralisation current flowing at the active electrode assemblies is measured and evaluated. The neutralisation efficiency and the residual charge remaining on the material web is calculated on the basis of the detected current. The speed of advance of the material web can then be adjusted depending on this residual charge.
A further antistatic device in which an active positive electrode assembly and an active negative electrode assembly are arranged consecutively in the direction of movement of the material web and wherein the neutralisation efficiency can be measured with the aid of the measured neutralisation currents of the active electrode assemblies is known from U.S. Pat. No. 6,259,591.
Such systems, in which both positive and negative electrodes are active during operation, can also be described as bipolar systems.
Active electrode assemblies differ from passive electrode assemblies in that the active electrode assemblies are connected to a high voltage source, whereas passive electrode assemblies are connected to a grounding element, particularly an earthing conductor. For the purposes of the present, high voltage is considered to be at least 1 kV.
For an active electrode assembly of such kind, it is generally possible to actuate the associated high voltage sources in order to generate an alternating current or a pulsed DC current, and in the case of pulsed DC current it is advantageous to actuate both high-voltage sources of a positive electrode assembly and a negative electrode assembly deliberately so that positive voltage pulses at the positive electrode assembly alternate with negative voltage pulses at the negative electrode assembly, thereby creating a virtual AC so to speak, if the two electrodes are considered as a single unit. However, the “zebra effect” is observed with alternating current and pulsed direct current. An unnecessary half-wave and half-polarity exist between each of two positive or negative voltage pulses, and neutralisation does not take place here because this half-wave has exactly the same polarity as the web and is therefore not available for discharging the material web and thereby avoiding personal injury, material damage, sparks, fire and explosions. Moreover, the voltage requires a certain time at the beginning of each voltage pulse until the ionisation voltage is built up. The ionisation voltage is the voltage level at which the ionisation of the surrounding air molecules begins at a charged peak. This ionisation phase based on a certain minimum voltage is absolutely necessary in order for the neutralisation to take place. The neutralising effect of the respective electrode assembly is reduced or even cancelled out entirely during these temporally delayed build-up and decay phases of the respective voltage pulse and during the unnecessary half-waves and polarities. Then, neutralised and non-neutralised or incompletely neutralised sections of the web may occur consecutively like zebra stripes depending on the speed of movement of the material web. The distances between these stripes, that is to say the bars of the zebra stripes, are correlated to the pulse frequency of the ionisation and the speed of the web.